When resistances, impedances, etc., of electronic components are measured, errors are produced in the measured values when the contacts between the device under test and the measurement terminals are imperfect. If poor contact occurs in the process of measuring characteristics of electronic component, there is a possibility that defective components will be shipped as good ones, and good components will be discarded.
In general terms, there are 2 methods for measuring impedance: 2-terminal measurements and 4-terminal measurements. The judgment of whether electrical contact is good or bad, in the case of a 2-terminal measurement, is generally made by the magnitude of the measurement results. In the case of a 4-terminal measurement, there are cases in which this judgment cannot be made solely from the magnitude of the measurement results.
An example may be understood by referring to FIG. 3 which is a diagram illustrating the principle of the 4-terminal measurement method. A high-voltage current terminal Hc and a high-voltage voltage terminal Hp (which are measurement terminals) contact one terminal 4 of device under test 3.
A low-voltage current terminal Lc and low-voltage voltage terminal Lp contact the other terminal 5 of device under test 3. Hc, Hp, Lc, and Lp constitute the 4 terminals. A signal source 1 is connected to high-voltage current terminal Hc through resistance R1, and current is thereby fed to device under test 3. The other terminal of the signal source is grounded or virtually grounded. Low-voltage current terminal Lc is also grounded or virtually grounded.
Measurement current flow is fed from signal source 1, through resistance R1 and measurement terminal Hc, to terminal 4 of device under test 3, and returns via terminal 5, through measurement terminal Lc, to signal source 1. The measurement current is measured by an ammeter in the loop. The current value may also be obtained by measuring the voltage between the ends of resistance R1. Another method is to connect an ammeter 9 to the Lc terminal. In this case, ammeter 9 may comprise an operational amplifier and a resistance to virtually ground the Lc terminal.
The voltage between the terminals of device under test 3, produced by the measurement current, is applied to input terminals 6 and 7 of differential amplifier 8 via high-voltage terminal Hp and low-voltage voltage terminal Lp, respectively, and is measured by a voltmeter 10 that is connected to the output terminal of differential amplifier 8. In ordinary impedance meters, the vector ratio of the values measured by voltmeter 10 and the ammeter 9 is obtained by a calculation process, and is output as the measured value.
Consider the case in which low-voltage voltage terminal Lp becomes open due to a poor contact. In this situation, the measurement current flows normally. Even if terminal Lp is open, when the potential of terminal 7 of differential amplifier 8 has a value close to ground potential, the measured value is close to the value obtained in a normal contact state. That is, contact quality cannot be judged by the magnitude of the measurement result. When this situation occurs in the testing of electronic components, there is a possibility that components that should be judged as being outside the standard and of poor quality will be judged as being of good quality.
In order to solve this problem, a contact judgment circuit and contact judgment method have been proposed in Japan Patent Application No. 3-308485. The invention disclosed therein adds a switch, a direct current source, and a direct current voltmeter to the impedance meter in order to judge the contact of terminal Lp. Therefore, the structure of the meter is more complicated, and its price is increased.